In the manufacture of microelectronic devices, such as processors, controllers, and memory, the desired structures are formed on a wafer. Individual dies are cut from the wafer and then sealed into a package. The package has an array of pins, pads, or lands that make contact with the rest of the device, typically through a printed circuit board to allow the die to be operated while within the package. Before packaging, either as part of a wafer or an individual die, each die is tested to determine whether it has been manufactured correctly and operates as intended.
To perform the tests, some wafers have lands or pads specifically designed into each die for test purposes. Pins or probes are applied to the lands or pads and signals are sent or received through the pins to test the dies. For a more complex die, more pins are used. Similarly for denser circuitry on the die, the pins are closer together. The distance between pads is called the pitch.
The test probes are attached to a substrate that holds each probe in the proper position with respect to each other probe so that when the test probes are pressed against the pads on the die, each probe will contact its respective pad. The probes are resilient and have a shaped tip so that when the probes are pressed against the die the pressure will stress and break through any oxide that has formed on the surface of the wafer die bumps in order ensure good electrical contact. The tips wear with each new contact and, as a result, each test probe has a limited number of die tests that it can perform, after which it must be replaced. The replacement cost of each test probe is therefore a factor in the cost of die manufacturing.
Test probes may be made out of composite wires. The cross section of such a composite wire has several layers of different metallic materials arranged coaxially around the wire's center. The metals perform different functions and not all of the metals are well-suited to be pressed against the pads. Some materials are formed of a high durability material. Other materials are selected for electrical properties rather than strength and may smear onto the pads under pressure, damaging the pad and the probe wire. As a result, the outer layers of the wires are removed before the probe wires are used. The probes wires, when attached to a probe card, are in a densely populated array with varying pitches.